1. Field of the Invention
The present invention relates to a spot welding system using a spot welding gun driven by a servomotor and a method of controlling a pressing force of the spot welding gun.
2. Description of Related Art
In performing a spot welding operation using a spot welding gun, objects of welding such as metal plates are clamped and pressed by a pair of welding tips of the spot welding gun and an electric current is flown between the welding tips through the objects to be fused together by Joule heat generated by the electric current. A spot welding gun using a servomotor for driving the welding tips is known as a servo welding gun.
FIG. 4 schematically shows a conventional welding system using the servo welding gun in which the welding tips are driven by a servomotor.
The spot welding gun 1 comprises arms 3a, 3b having welding tips 2a, 2b respectively at distal ends thereof, a ball screw/nut mechanism 4 for converting a rotary motion to a linear motion, and a servomotor 5 for driving the arm 3a relative to the arm 3b through the ball screw/nut mechanism 4 to open/close the welding tips 2a and 2b for clamping and applying a pressing force on objects of welding. A position detector 6 is provided at the servomotor 5 to detect a rotational position of the servomotor 5 and thus linear position of the welding tips 2a, 2b. In general, a controller 10 for controlling the servo spot welding gun 1 is constituted by a robot controller for controlling a robot to which the welding gun 1 is attached. Specifically, a servo CPU 12 for controlling the servomotor 5 is additionally provided in the robot controller or a servo CPU provided in the robot controller for controlling servomotors for axes of the robot may be utilized as the servo CPU 12 for controlling the servomotor 5 of the spot welding gun 1.
A main CPU 11 of the controller 10 issues a command for operating a robot to move the spot welding gun 1 to a predetermined welding position and then issues a motion command to the servomotor 5 of the spot welding gun 1. The servo CPU 12 performs position feedback processing based on the motion command and feedback signals from the position detector 6 and outputs current commands (torque commands) for the servo amplifier 13. The servo amplifier 13 drives the servomotor 5 to move the arm 3a relative to the arm 3b by flowing a driving current in the servomotor 5 in accordance with the current command so that the welding tips 2a and 2b move closer to each other.
When the welding tip 2a is moved to a predetermined position relatively to the welding tip 2b, the main CPU 11 issues a control changeover command and a pressing force command to the servo CPU 12. Upon receipt of these commands, the servo CPU 12 switches the position feedback control to a pressing force control and issues a current command (torque command) to the servo amplifier 13 for driving the servomotor 5 so that a commanded pressing force is applied to the objects by the welding tips 2a and 2b. Then, a welding current is flown between the welding tips 2a and 2b through the objects to fuse the objects together, and after the welding the welding tips 2a and 2b are opened and the welding gun 1 is moved to a next welding position.
As described, the servomotor 5 for driving the welding tips 2a and 2b is arranged relatively closer to the welding tips 2a and 2b in the welding gun 1. The Joule heat generated by the welding current flown between the welding tips 2a and 2b through the objects rises temperature of the servomotor 5 and temperature of movable components of a drive force transmission mechanism including the arm 3a, 3b and the ball screw/nut mechanism 4 of the spot welding gun 1 to a high degree. For reducing the high temperature, the distal ends of the arms 3a, 3b and the welding tips 2a, 2b of the spot welding gun 1 are regularly water cooled, and the servomotor is water cooled in some spot welding guns.
In general, an output torque of an electric motor decreases as temperature thereof increases. An output torque of the electric motor is expressed by the following equation.Output torque=torque constant×electric current
It is known that the torque constant varies in dependence on temperature of the motor so that it decreases as the temperature increases from the disclosure of JP 2002-165478 A, for example.
Since the temperature of the servomotor of the spot welding gun is increased by the Joule heat generated by the welding current and heat generated by the motor itself, the output torque decreases with the decrease of the torque constant caused by the high temperature, so that the pressing force of the welding tips is lowered.
The variation of the torque constant with variation of the temperature of the servomotor caused by the heat generation of the motor itself is not so large to be negligible, but the temperature rise of the servomotor caused by the Joule heat generated by the welding current are large. Thus, the torque constant is decreased to lower the torque of the servomotor 5 for driving the welding tips 2a, 2b. 
Further, in the servo spot welding system, the servo amplifier 13 flows the driving current in the servomotor 5 to produce an output torque in accordance with the current command from the servo CPU 12 and the output torque is converted from rotary motion to linear motion by a motion converting mechanism such as the ball screw/nut mechanism 4 to linearly move the welding tips 2a, 2b so that a pressing force is applied on the objects, as shown in FIG. 5.
In converting the rotary motion to the linear motion to produce a linear force, a relation between the output torque of the motor and the converted linear force is ideally expressed as follows;Force(N)=Torque(Nm)/(an amount of linear motion per one rotation of the motor×2π).
In practice, the above relation is not established because of energy loss caused by friction, etc. It is known that a friction loss depends on temperature of movable components of the motion converting mechanism.
Thus, the servo spot welding system is influenced by the Joule heat generated by the welding current such that the torque constant of the servomotor 5 for driving the welding tips 2a, 2b varies (the torque constant is lowered with temperature rise) and the friction loss of components of a driving force transmission mechanism including the ball screw/nut mechanism and the arms varies with the variation of temperature (the friction loss is lowered with temperature rise). Therefore, there arises a problem of failing in applying a constant pressing force on the objects of welding by the welding tips in accordance with a commanded pressing force by complex effect of the variation of the torque constant and the variation of the friction loss with the high temperature.